1. Field of the Invention
The present invention relates to a power conversion apparatus using a semiconductor switching device.
2. Description of the Background Art
For example, a configuration shown in FIG. 8 is known as a power conversion apparatus of such a type.
The power conversion apparatus shown in FIG. 8 has a single-phase AC power supply 101, a rectifier 102 for converting AC output power of the AC power supply 101 into DC power, a smoothing capacitor 103 for smoothing the DC power outputted from a positive-side line Lp and a negative-side line Ln of the rectifier 102, and a DC load 104 connected between the positive-side line Lp and the negative-side line Ln.
The rectifier 102 has a configuration in which series circuits 107 and 108 are connected in parallel between the positive-side line Lp and the negative-side line Ln. In each series circuit 107, 108, a diode 105a, 105b serving as a rectifying device and, for example, a MOSFET 106a, 106b serving as a semiconductor switching device are connected in series. Here, since each MOSFET 106a, 106b has a body diode internally, each MOSFET 106a, 106b is always conductive as to a reverse current regardless of its gate voltage.
Each AC input point Pia, Pib corresponding to a connection point between the diode 105a, 105b and the MOSFET 106a, 106b in each series circuit 105, 106 is connected to the AC power supply 101 through an inductor 109a, 109b. 
In addition, a series circuit of grounded capacitors 110a and 110b serving as a noise filter is connected between power supply lines La and Lb which connect the inductors 109a and 109b to the output side of the AC power supply 101 respectively. A connection point between the grounded capacitors 110a and 110b is connected to ground potential G.
The power conversion apparatus shown in FIG. 8 has a function by which an AC input current Iin of the AC power supply 101 inputted into the rectifier 102 can be formed into a sine waveform whose phase is equal to that of the AC input voltage Vin while a DC output voltage Ed is kept at a desired value which is higher than the peak value of the AC input voltage Vin.
Description will be made below of the operation for implementing such a function.
For example, assume that the AC input voltage Vin is positive. In this case, when the MOSFET 106a is turned on, a current flows in a path from the AC power supply 101 back to the AC power supply 101 via the power supply line La, the inductor 109a, the MOSFET 106a, the MOSFET 106b, the inductor 109b and the power supply line Lb. Thus, the voltage of the AC power supply 101 is applied to the inductors 109a and 109b half-and-half (e.g., in an even split) to increase the AC input current Iin.
When the MOSFET 106a is turned off in this state, the current flows in a path from the AC power supply 101 back to the AC power supply 101 via the power supply line La, the inductor 109a, the diode 105a, the smoothing capacitor 103, the MOSFET 106b, the inductor 109b and the power supply Lb. On this occasion, a differential voltage between the DC output voltage Ed and the AC input voltage Vin is applied to the inductors 109a and 109b half-and-half. Since the DC output voltage Ed is kept higher than the peak value of the AC input voltage Vin due to the operation of the circuit, the AC input current Iin is reduced.
Accordingly, when a ratio between on-time and off-time (i.e. a duty ratio) of the MOSFET 106a is controlled, the waveform and magnitude of the AC input current Iin can be controlled desirably. Thus, the waveform of the AC input current Iin can be formed into a sine waveform (taking no account of a ripple component here). In addition, when the amplitude of the AC input current Iin is controlled in accordance with load power, the DC output voltage Ed can be kept at a desired value.
When the AC input voltage Vin is negative, a similar operation is performed by on-off operation of the MOSFET 106b. Here, the MOSFET 106b is conductive reversely regardless of its gate signal when the AC input voltage Vin is positive (the MOSFET 106a performs on-off operation), and the MOSFET 106a is conductive reversely regardless of its gate signal when the AC input voltage Vin is negative (the MOSFET 106b performs on-off operation).
Generally in the power conversion apparatus performing switching thus, noise is generated due to fluctuation of potential provided every switching. The noise is prevented from flowing to the outside by the grounded capacitors 110a and 110b as a noise filter. Here, when the neutral point potential of the AC input voltage Vin corresponds to the ground potential G, the voltage of each grounded capacitor 110a, 110b is Vin/2. FIG. 9 shows a change of potential at each point relative to the ground potential G due to switching. At the timing when the MOSFETs 106a and 106b are on, points U and V at the opposite ends of the grounded capacitors 110a and 110b are short-circuited, and voltages VL1 and VL2 of the opposite ends of the inductors 109a and 109b are expressed by VL1=VL2=Vin/2 as described previously. Here, since the potential at the point U is +Vin/2 and the potential at the point V is −Vin/2, the potential at each AC input point Pia, Pib is 0 V. Since the MOSFETs 106a and 106b are conductive, the potential at the AC input point Pia, Pib is also equal to the potential at a negative-side point N of the smoothing capacitor 103. Hence, the potential at the point N is 0V. The potential at a positive-side point P of the smoothing capacitor 103 is equal to the sum of the potential at the point N and the DC output voltage Ed. Hence, the potential at the point P is equal to the DC output voltage Ed.
On the other hand, at the timing when the AC input voltage Vin is positive and the MOSFET 106a is turned off, the potential at the AC input point Pia is equal to the potential at the point P, and the potential at the AC input point Pib is equal to the potential at the point N. Accordingly, the DC output voltage Ed is applied between the points U and V at the opposite ends of the grounded capacitors 110a and 110b. Hence the relation of VL1=VL2=(Vin−Ed)/2 is established. Thus, the potential at the point Pia (=the potential at the point P) is expressed by Vin/2−(Vin−Ed)/2=+Ed/2, and the potential at the point Pib is expressed by −Vin/2+(Vin−Ed)/2=−Ed/2. That is, the potential at the point Pia fluctuates by +Ed/2, and the potential at each point Pib, P, N fluctuates by −Ed/2.
Due to the same operation, at the timing when the AC input Voltage Vin is negative and the MOSFET 106b is turned off, the potential at the point Pib fluctuates by +Ed/2 and the potential at each point Pia, P, N fluctuates by −Ed/2. When the MOSFETs 106a and 106b are turned on again, potential fluctuation is reversed.
In the background-art example shown in FIG. 8, there are unintended parasitic capacitances 111, 112, 113 and 114 between each point and a frame FG of the apparatus. Since the frame FG is grounded for safety reasons, the parasitic capacitances 111 to 114 act as earth capacitances. As a result of the aforementioned potential fluctuation, a current flows into each parasitic capacitance to circulate a current Ie in the circuit through the grounded capacitors 110a and 110b as shown in FIG. 8. On this occasion, a so-called noise terminal voltage which is a high-frequency voltage is generated in each grounded capacitor 110a, 110b. 
The noise terminal voltage must be limited in order not to give bad influence to another apparatus connected to the AC power supply 101. It is the simplest method to increase the capacitances of the ground capacitors 110a and 110b. However, the grounded capacitors 110a and 110b permit not only a high-frequency current but also a low-frequency current derived from the AC input voltage Vin as a leak current. Therefore, the capacitances made too high may lead to problems such as earth leakage circuit breaker tripping. There is another well-known method in which a common mode choke coil is inserted between the apparatus and the power supply. However, the common mode choke coil, which must allow a main circuit current to flow therein, is bound to increase in its outer shape. Thus, the common mode choke coil prevents a power conversion apparatus from being miniaturized, and further leads to cost increase.
On the other hand, as a noise suppressing method using a switching unit, there is known a noise reduction apparatus as follows (e.g. see JP-A-2002-119065). That is, in the noise reduction apparatus, for example, a current supply circuit including a switching device for generating a current in a reverse direction to a noise current detected by a noise detection unit is connected between a voltage clamp circuit using an energy regenerating transformer connected to a smoothing capacitor and ground potential G so as to suppress noise.
As another noise suppressing method, there is known an electric vehicle as follows (e.g. see JP-A-2009-33891). That is, in the electric vehicle, an electrostatic capacitance is connected in series with an intrinsic stray capacitance between a high-voltage system component including an inverter or a motor generator and the vehicle earth, and a connection switch is connected in parallel to the electrostatic capacitance so that the connection switch is controlled to be on when a load is driven, and to be off when the load is not driven.
However, in the background-art example disclosed in JP-A-2002-119065, the switching unit itself must produce the current to cancel the noise. Thus, there remains an unresolved problem that the configuration of the switching unit becomes complicated because the switching unit has to switch at an extremely high speed and, at the same time, has to be controlled at a high speed.
On the other hand, in the background-art example disclosed in JP-A-2009-33891, the earth capacitance is changed in accordance with the connection condition of the apparatus as a whole to the outside, so as to adjust the potential fluctuation of the apparatus and the magnitude of the leak current. There remains an unresolved problem as follows. That is, the background-art example cannot be applied when the connection condition of the apparatus as a whole to the outside is fixed, and further, the leak current cannot be reduced when the apparatus serving as a noise source is operating.